In these years, a light called discharge light is used for a head light on an automotive vehicle. A lamp, i.e., a discharge lamp, which emits light by discharging electricity, provides the discharge light. Characteristics (e.g., electricity consumption, brightness, and lifetime) of the discharge lamp on the vehicle are improved dramatically compared with a conventional lamp using a filament. The conventional discharge lamp on the vehicle is disclosed in U.S. Pat. No. 6,850,015, for example.
The conventional discharge lamp on the vehicle includes a discharge lamp, which emits light, and a power controller for supplying and controlling a power for the discharge lamp. The conventional discharge lamp is supplied with different electric powers between at a start-up of lighting and at a stationary state coming after the start-up. That is, at the start-up, a large power is needed for the discharge lamp, in order to generate the discharge between electrodes in the discharge lamp and provide light having required intensity immediately after the start of lighting. Actually, the large power (i.e., starting electricity) is provided for the discharge lamp. Next, after the discharge generation in the discharge lamp, maintaining a stable discharging state is needed so that a stable power (i.e., stationary electricity) is supplied to the discharge lamp. For example, the discharge lamp functions with a condition that the starting electricity is 75 W and the stationary electricity is 35 W. The stationary electricity is lower than the starting electricity. The power controller controls the power supplied to the discharge lamp. A shift from the starting electricity to the stationary electricity is performed in such a manner that the shift is provided by a linear curve having a large gradient. The large gradient represents that an electricity variation per unit time becomes large.
After a state, in which the discharge lamp is not lighting, is kept for a long time, when the discharge lamp starts lighting (i.e., a cold start is performed), discharging electrodes are not warmed adequately. Under this condition, when the electricity is lowered rapidly from the starting electricity to the stationary electricity, a temperature irregularity is generated on the electrodes. The temperature irregularity on the electrodes causes a partial change of the discharging place on the electrodes. The partial change of the discharging place on the electrodes causes a change of an arc, which is generated between electrodes of the discharge lamp. The change of the arc causes a fluctuation of the arc. Thus, the light of the discharge lamp is not stable.
Moreover, in the conventional discharge lamp on the vehicle, a radio noise is generated by the discharge in the discharge lamp and the rapid electricity change.